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Crystallization, a fundamental process in nature, hinges on two distinct stages: nucleation and growth. The latter plays a pivotal role in shaping the morphology, size, and purity of crystalline materials, making it a focus for scientific research and defect engineering. Understanding how crystals grow at the atomic level has long been a key challenge in the field.

To address this challenge, a collaborative research team from the Xinjiang Technical Institute of ÌÇÐÄÊÓƵics and Chemistry of the Chinese Academy of Sciences, along with researchers from the Lawrence Livermore National Laboratory in the U.S. and the International Iberian ÌÇÐÄÊÓƵ Laboratory in Portugal, has revealed at the atomic scale how factors such as size, defect density, and approach pathways influence crystal growth through coalescence. Their findings were in the Journal of the American Chemical Society.

Using advanced in-situ dynamic imaging with an aberration-corrected transmission electron microscope (AC-TEM)—a tool that enables atomic-level precision—the researchers studied the post-nucleation growth of five-fold twinned (5-FT) gold nanocrystals as they coalesced, or merged, into larger structures.

Their observations revealed two primary pathways through which these 5-FT gold nanocrystals grow via coalescence:

• When two small 5-FT nanocrystals (each measuring 6–11 nm) merge, the process is driven by "de-twinning"—a mechanism that reduces internal structural defects.
• When a 5-FT nanocrystal merges with another nanocrystal (of unspecified structure), growth is dominated by atomic rearrangement and surface migration.

The study also found that once merged particles exceed a critical size, they tend to form complex multi-twinned structures.

Crucially, the researchers identified factors beyond particle size that govern coalescence: Both the initial density of planar defects and the specific pathways by which nanocrystals approach one another impact growth rates and the final structure of the resulting crystal.

Additionally, the team documented detailed atomic-scale dynamics of the coalescence process, including intermediate structures, step-by-step grain boundary migration, and the formation or elimination of twinned regions (twinning and de-twinning).

These insights provide valuable atomic-level details for advancing materials science and defect engineering applications.